High Capacity Li-rich Layered Solid Solution Cathode Materials For Li-ion Battery

In recent years, in the background of shortage of global oil resources and the worseningof the environment, lithium ion battery began to be used in electric cars, energy storage powerstation and intelligent power grid, etc, so as to reduce dependence on oil and the relief of airpollution. For such applications, lithium ion batteries must have high energy density andpower density with good safety and environmental friendliness. But current positive materialsare hard to meet the high density and high power density requirements. In this context, thelayered Li-rich Li_1+zM_1-zO₂（M is one or more than one type of transition metal elements, z≥0）material has been the research focus because of its high capacity, excellent cycleability andnew charge and discharge mechanism. They can be the most possible candidate of cathodematerials for next-generation Li-ion battery （300 Wh/kg）.The main contents of this thesis, centered on the tetrahedral phase diagram of LiNiO₂-LiCoO₂-LiMnO₂-Li₂MnO₃, are as follows:Firstly, with the aid of a tetrahedral phase diagram of LiNiO₂-LiCoO₂-LiMnO₂-Li₂MnO₃, wedesigned a series of new cathode materials of Li_1+zM_1-zO₂(M=Ni_1-m-nCo_mMn_n，z=_0.2), whichcan be rewritten as Li_1.2(Co_0.4Mn_0.4)1-x-y(Ni_0.4Mn_0.4)x(Ni_0.2Mn_0.6)yO₂（0≤x+y≤1） solid solutions.Exploratory synthesis of those designed materials via a simple combustion method was alsoattempted, followed by preliminary structural and electrochemical characterizations. XRD andSEM results showed that calcining temperature has important effects on the structure andmorphology of materials. All the materials prepared at 950℃were a pure phase withwell-developed layered structure，which well supported our judgement on the tetrahedral phasediagram of LiNiO₂-LiCoO₂-LiMnO₂-Li₂MnO₃. Preliminary electrochemical characterizationsindicated that materials can exhibit different electrochemical properties depending onmaterials composition.Secondly, two compositions of Li_1.2(Co_0.4Mn_0.4)1-x-y(Ni_0.4Mn_0.4)_x(Ni_0.2Mn_0.6)_yO₂（0≤x+y≤1）series materials (Li_1.2Ni_0.13Co_0.13Mn_0.54O₂and Li_1.2Ni_0.15Co_0.2Mn_0.45O₂) were selected toinvestigate the effect of synthesis route on materials performances. A hydroxideco-precipitation route was used to optimize materials synthesis and compared with thecombustion method of acetates. Comparative study indicated that the materials synthesized byco-precipitation method presented higher crystallinity with better layered characteristics,higher charge/ discharge capacity and slightly improved cycleability than those prepared bycombustion method of acetates. Further investigation of rate performance on the materialsprepared via co-precipitation route demonstrated good rate capability of materials andimproved cycleability with increased rate.Finally, considering the possible important impacts of Li content on performances oflayered Li-rich Li_1+zM_1-zO₂（M is one or more than one type of transition metal elements, z≥0）materials, two hydroxide precursors of the materials with compositions of x=0, y=1 and x=0, y=0.8 in Li_1.2(Co_0.4Mn_0.4)1-x-y(Ni_0.4Mn_0.4)x（Ni0.2Mn0.6）yO₂（0≤x+y≤1） solid solutions wereselected as targets to investigate the effects of Li content vs hydroxide precursors on materialssynthesis, structure and electrochemistry. The obtained materials can be rewritten asLiδNi_0.25-zMn_0.75-zCo_2z]O_y（0≤δ≤2，z=0 and 0.05）. The results showed that, along with thechange ofδvalue, the structure, morphology and electrochemical properties of materialsexhibited remarkable changes. With the increasing ofδ, the materials structure presented adramatic change from a pure spinel phase to the spinel/layered mixed phase and then to a purelayered phase. It can be preliminarily deduced that spinel phase can form in the case of0≤δ≤0.5, the mixture of spinel and layered phases coexist in the range of 0.5<δ<1.5, and atδ≥1.5 layered structure can be developed easily. These results further well supported ourjudgement on the tetrahedral phase diagram of LiNiO₂-LiCoO₂-LiMnO₂-Li2MnO3.